A high school student, Dave Lizewski, is an unnoticed teenager with a love for comic books and a desire to make a difference. Tired of feeling powerless and witnessing rampant crime in the city, he decides to become a real-life superhero, despite having no powers or training. He purchases a costume online, dubs himself "Kick-Ass," and sets out to fight crime. His first attempt at vigilantism is a disastrous failure, resulting in him getting stabbed and hit by a car. However, this accident gives him slightly enhanced capabilities due to metal implants and damaged nerve endings. Dave persists in his quest, gaining public attention when a video of him fighting thugs goes viral.

Meanwhile, Big Daddy, a former cop, has trained his eleven-year-old daughter, Hit-Girl, to be a deadly vigilante after his life was destroyed by a powerful crime boss. The crime boss's son, eager to impress his father, creates another alter-ego, Red Mist, in an attempt to gain favor and find and stop Kick-Ass. As the paths of these disparate individuals cross, Kick-Ass finds himself part of a violent and complex world of crime-fighting far more dangerous and real than he ever anticipated. He is confronted with a labyrinth of crime and moral ambiguity that challenges his initial naive understanding of what it means to be a hero.